oops. Include btkart.hpp to world.cpp so it compiles again.

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/branches/irrlicht@3887 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/modes/world.cpp

@@ -40,6 +40,7 @@
 #include "karts/kart_properties_manager.hpp"
 #include "network/network_manager.hpp"
 #include "network/race_state.hpp"
+#include "physics/btKart.hpp"
 #include "race/highscore_manager.hpp"
 #include "race/history.hpp"
 #include "race/race_manager.hpp"